1. Field of the Invention
The present invention relates to a high density nickel hydroxide containing boron and a method for preparing it; the invention is particularly useful for alkali rechargeable batteries. According to the process of the invention, factors including pH are controlled while supplying a nickel sulfate solution containing a boron compound, a sodium hydroxide solution and ammonium hydroxide to produce high density nickel hydroxide containing boron.
2. Description of the Related Art
Due to recent developments in the electronics industry, the demand for portable electronics such as wireless phones, camcorders, and portable computers has greatly increased. In accordance with this trend, the demand for batteries has likewise increased. As electronic devices are getting smaller and lighter with increasingly demanding performance requirements, a battery with high energy density and high capacity is needed. In response, great advances in the performance of Ni-Cd and lead acid batteries have been realized. However, as a result of cadmium and lead being known pollutants, environmental concerns have increased regulation on the use of these materials. Therefore, there is a significant need for the development of a pollution free rechargeable battery. An increase in the regulation of vehicle emissions has also increased the need for development of a pollution free vehicle.
Rechargeable batteries which meet these needs include Ni-MH (Metal Hydride) batteries, Ni-Fe batteries and Ni-Zn batteries. These batteries replace the cadmium negative electrode in a conventional Ni-Cd battery with Metal Hydride, Fe and Zn respectively. Since these batteries are pollution free and have higher theoretical capacities than Ni-Cd batteries, they are the current focus of research and development. Recently, a small size Ni-MH battery has been commercialized and is in the initial stages of mass production. However, other batteries have yet to be commercialized. A nickel hydroxide electrode which is used in the conventional Ni-Cd battery is also used as the positive electrode in these rechargeable batteries. The development of high capacity nickel hydroxide electrodes is a key to the future commercialization of these rechargeable batteries. The rechargeable battery using a conventional nickel hydroxide electrode mentioned above has a capacity lower than the capacity of a battery using the new negative electrode materials. In order to develop a high capacity battery resulting from use of the new negative electrode materials, production technology of the active materials must coincide with advances in electrode fabrication technology.
The properties of nickel hydroxide used as an active material in a nickel hydroxide electrode differ depending on the production method. Generally, nickel hydroxide is produced using the neutralization method in which nickel salt and hydroxide salt are mixed and then a small amount of water is added. The precipitate particles are so coarse and their size distribution is so wide (ranging from 1 to hundreds of microns) that it is often only usable after being pulverized. The precipitate's irregular shape and low density render it inappropriate for use in a battery. When it is neutralized in solution, the rate of reaction is so fast that the precipitate is too fine and the density is too low, which requires longer filtering or washing times and increases the adsorption of water on the surface. As a result, it is difficult to load large amounts of active material on an electrode and it is therefore impossible to increase the electrode's capacity.
Nickel hydroxide used for pasted nickel hydroxide electrodes should be spherical, have high density, and have a narrow size distribution. Nickel hydroxide appropriate for a battery should have an apparent density of 1.4-1.7 g/cm.sup.3, a tapping density of 1.8-2.1 g/cm.sup.3, and a size range of 5-50 .mu.m. Since the paste made with this kind nickel hydroxide has excellent fluidity and uniformity, it is possible to fabricate high capacity and uniform electrodes. The use of this kind of nickel hydroxide also improves the utilization of the active material and discharge capacity of the electrode.
In order to produce high density spherical nickel hydroxide, it should be grown gradually. Nickel ions first form complex ions with ammonia and then neutralize or raise the temperature of the solution, thus reducing and controlling the reaction rate so that nickel hydroxide is gradually precipitated by decomposition of the nickel ammonium complex. Nickel hydroxide precipitated in this manner has a high density, but it is difficult to control the reaction rate and particle size; the fluctuation of the composition and the deviation of the solution's pH during the reaction render its stable and continuous production difficult.
Electrode expansion is known to be the main cause of pasted nickel hydroxide electrode degradation. Such expansion occurs because .beta.-NiOOH transforms into low density .gamma.-NiOOH. This expansion breaks down the active material and degrades the conductivity of the electrode, which in turn rapidly decreases cycle life and capacity. The low density .gamma.-NiOOH is characterized by an unstable crystal structure.
Protons do not transfer easily when high density nickel hydroxide is used. When charging at a constant current, a higher overpotential is needed near the end of the charge. Thee already charged .beta.-NiOOH is constantly oxidized and transforms into low density .gamma.-NiOOH having a higher level of oxidation. When the low density .gamma.-NiOOH is formed, the active material's volume increases, resulting in the expansion of the electrode. As charge and discharge cycles are repeated, the electrode material breaks down due to changes in volume; conductivity significantly deteriorates and capacity abruptly decreases. High rates of charge and discharge exacerbate these problems.
It is reported that additives such as Co, Zn, Cd are effective to prevent formation of low density .gamma.-NiOOH. Substitution of some of the Ni with these elements causes a distortion of the lattice which increases the mobility of protons during charge and discharge reactions in the positive electrode and decreases overvoltage reactions.
In this invention, by adding boron as an active material which does not substitute for the Ni ions, high density nickel hydroxide containing boron may be continuously produced. Boron is effective to prevent the formation of low density .gamma.-NiOOH. The specific surface area of nickel hydroxide coprecipitated with boron is increased 2-3 times more than that of pure nickel hydroxide, which is expected to increase the high rate discharge capacity. Furthermore, by adding Co, Zn and Cd as well as boron as active material additives to the nickel sulfate solution, high density nickel hydroxide containing boron and Co, Zn, Cd is produced, thus maximizing utilization of active material and preventing the expansion of the electrode. This method thereby produces a high capacity nickel hydroxide electrode.
Accordingly, it is an object of the present invention to provide a high density nickel hydroxide containing boron and a method for preparing it. The high density nickel hydroxide has excellent quality and may be produced by the continuous supply process of reactants and the continuous overflow of product. The high density nickel hydroxide of the invention is particularly useful for alkali rechargeable batteries.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.